Structure and Function of the Musculoskeletal System

Functions of the Skeletal System
Serves as a framework for the attachment of muscles, tendons, and ligaments
Protects and maintains soft tissues in their proper position
Provides stability for the body
Maintains the body’s shape
Acts as a storage reservoir for calcium
Contains the hematopoietic connective tissue in which blood cells are formed

Structure of the Skeletal System
The axial skeleton
Composed of the bones of the skull, thorax, and vertebral column
Forms the axis of the body
The appendicular skeleton
Consists of the bones of the upper and lower extremities, including the shoulder and hip
Types of Connective Tissue Found in the Skeletal System
Cartilage
Bone
Each of these connective tissue types consists of: living cells, nonliving intercellular protein fibers, an amorphous (shapeless) ground substance

Types of Intercellular Fibers Found in Skeletal Tissue
Collagen
An inelastic and insoluble fibrous protein
Has great tensile strength
Tissues that contain large numbers of collagenous fibers generally appear white
Elastin
The major component of elastic fibers that allows them to stretch several times their length and rapidly return to their original shape when the tension is released
Cartilage
Definition
A firm but flexible type of connective tissue
Composition
Cells and intercellular fibers embedded in an amorphous, gel-like material
Types
Elastic cartilage
Hyaline cartilage
Fibrocartilage
Bone
Definition
Connective tissue in which the intercellular matrix has been impregnated with inorganic calcium salts
Has great tensile and compressible strength but is light enough to be moved by coordinated muscle contractions
Composition
Two types of substances—organic matter and inorganic salts
Types of Bone
Cancellous (spongy) bone
Found in the interior of bones
Composed of trabeculae, or spicules, of bone that form a lattice-like pattern
Compact (cortical) bone
Forms the outer shell of a bone
Has a densely packed calcified intercellular matrix that makes it more rigid than cancellous bone
Types of Bone Cells
Osteogenic cells
Osteoblasts
Osteocytes
Osteoclasts

Actions of Parathyroid Hormone
Increases intestinal absorption of calcium
Increases intestinal absorption of phosphate
Decreases renal excretion of calcium
Increases renal excretion of phosphate
Increases bone resorption
Decreases bone formation
Promptly increases serum calcium levels
Prevents increase in serum phosphate levels
Action of Calcitonin
Increases renal excretion of phosphate
Increases renal excretion of calcium
Decreases bone resorption
Decreases serum calcium levels with pharmacologic doses
Decreases serum phosphate levels with pharmacologic doses

Action of Vitamin D
Increases intestinal absorption of calcium
Increases intestinal absorption of phosphate
Increases renal excretion of phosphate
Can increase bone resorption
Can increase bone formation

Classification of Bones
Long bones
Found in the upper and lower extremities
Short bones
Irregularly shaped bones located in the ankle and the wrist
Flat bones
Composed of a layer of spongy bone between two layers of compact bone
Found in areas such as the skull and rib cage
Articulations or Joints
Definition
Areas where two or more bones meet
Classes of joints
Based on movement and the presence of a joint cavity
Synarthroses: lack a joint cavity; move little or not at all
Diarthroses: are freely movable